1. Field of the Invention
This invention relates to a system and method for monitoring and surveillance, and particularly to a system and method employing both wide-angle and narrow-angle imaging optics.
2. Discussion of the State of the Art
Traditionally, surveillance and monitoring ("SAM") systems have used off-the-shelf lenses and cameras for imaging. Because of the limitations of lenses, these systems typically provide very limited fields of view. To increase their limited fields of view, traditional SAM systems have relied on panning and tilting of the imaging system. As defined in this specification and the appended claims, "panning" refers to movement in a plane substantially horizontal to the area being monitored, and "tilting" refers to movement in a plane substantially vertical to the area being monitored. Typically, when a pan-and-tilt system is used, the system scans an area in some predefined or random path until an object of interest is detected. At that point, the object may be tracked by the system for further observation. Typically, such a system may also include a zoom lens for zooming in on objects of interest. Such systems are usually characterized as pan, tilt, and zoom ("PTZ") systems. Another approach to SAM systems has been the use of multiple PTZ systems to cover an area being monitored. Examples of PTZ systems are described in U.S. Pat. No. 5,627,616 to Sargeant et al.; U.S. Pat. No. 5,394,209 to Stiepel et al.; U.S. Pat. No. 5,164,827 to Paff; and U.S. Reissue Pat. No. 34,989 to Struhs et al., which are incorporated herein by reference.
While PTZ systems enlarge the field of view capable of being monitored by a lens-and-camera system, their scanning time makes them unsuitable for many real-time applications - - - i.e., fast-moving objects may enter and leave the area being monitored before the PTZ systems are able to detect them. Additionally, if a PTZ system uses a predefined scanning path, the monitoring performed by the system could be circumvented. That is, if an intruder is aware of the predefined scanning path, the intruder may be able to move about the monitored area without being detected by the PTZ system.
Another approach to SAM systems has been the use of wide-angle or so-called "omnidirectional" imaging systems. For example, the use of "fish-eye" lens for wide-angle viewing is disclosed in E. L. Hall et al., "Omnidirectional Viewing Using a Fish Eye Lens", SPIE Vol. 728 Optics, Illumination, and Image Sensing for Machine Vision (1986), p. 250, and U.S. Pat. No. 5,185,667 to Zimmerman, which is incorporated herein by reference. Since the fish-eye lens has a very short focal length, the field of view may be as large as or sometimes greater than a hemisphere.
Other prior art devices have used reflecting surfaces to increase the field of view. One such prior art device is disclosed in V.S. Nalwa, "A True Omni-Directional Viewer", AT&T Bell Laboratories Technical Memorandum, BL0115500-960115-01, January 1996. Nalwa discloses the use of multiple planar reflecting surfaces in conjunction with multiple charge-coupled device ("CCD") cameras to obtain a 360 degree panoramic image of a 50 degree band of a hemispherical scene. Specifically, in Nalwa, four planar mirrors are arranged in the shape of a pyramid, with one camera being positioned above each of the four planar reflecting sides, and with each camera viewing slightly more than 90 degrees by 50 degrees of the hemispherical scene. A similar device is disclosed in U.S. Pat. No. 5,539,483 to Nalwa, which is incorporated herein by reference.
Both Yagi et al., "Evaluating Effectivity of Map Generation by Tracking Vertical Edges in Omnidirectional Image Sequence", IEEE International Conference on Robotics and Automation, June 1995, p. 2334, and Yagi et al., "Map-Based Navigation for a Mobile Robot With Omnidirectional Image Sensor COPIS", IEEE Transactions on Robotics and Automation, Vol. II, No. 5, October 1995, disclose a conical projection image sensor (COPIS) which uses a conical reflecting surface to gather images from the surrounding environment and processes the information to guide the navigation of a mobile robot.
Yamazawa et al., "Obstacle Detection With Omnidirectional Image Sensor HyperOmni Vision", IEEE International Conference on Robotics and Automation, October 1995, p. 1062, discloses a purported improvement to the COPIS system which involves the use of a hyperboloidal reflecting surface in place of a conical surface. Prior to Yamazawa et al., U.S. Pat. No. 3,505,465 to Donald Rees also disclosed the use of a hyperboloidal reflecting surface to achieve panoramic viewing. Rees is incorporated herein by reference.
As compared to traditional PTZ systems, the wide-angle or omnidirectional prior art devices described above have certain disadvantages. For example, the wide-angle or omnidirectional devices typically provide image resolution that is relatively low as compared to traditional PTZ systems. This is because, to avoid costly special designs, the wide-angle or omnidirectional devices typically utilize off-the-shelf cameras whose resolution is adequate for smaller fields of view. In addition, a lower resolution is often necessary if real-time video images are desired from the wide-angle or omnidirectional devices because real-time, high resolution video images of a wide field of view require a great amount of throughput on the part of image processing equipment connected to the wide-angle or omnidirectional devices. Moreover, a further drawback of wide-angle or omnidirectional devices as compared to traditional PTZ systems is that zooming in on a region of interest by image processing of a wide-angle or omnidirectional image cannot provide better resolution of the region of interest than in the original wide-angle or omnidirectional image, whereas zooming in on a region of interest with a zoom lens of a traditional PTZ system can provide higher resolution of the region of interest than in the original image.
Accordingly, there exists a need for a cost-effective SAM system that provides both a wide-angle field of view of an area being monitored in combination with the capability for high-resolution images of regions of interest within the monitored area.